Fails when changing WhatsApp/Messenger notification sound

I just upgraded to MM (Slimm ROM from LP Slimm), and my custom WhatsApp and Facebook Messenger notification stopped working. When I try to set them again, I find that:

In WhatsApp, I can go to the notif sound setting and select the custom sound I want. But if I change the sound the new setting is not registered, and reverts back to system default. However if I change the setting to one of the stock sounds it will work properly.

In Facebook Messenger, I can't see my custom sounds at all.

I'm asking for both of these because I suspect the two problems may be related. Is there perhaps some permission settings I could change?